Audio coding technology that compresses the amount of data of an audio signal or an acoustic signal to one-several tenths of its original size is significantly important in the context of transmitting and accumulating signals. One example of widely used audio coding technology is transform coding that encodes a signal in a frequency domain.
In transform coding, adaptive bit allocation that allocates bits needed for encoding for each frequency band in accordance with an input signal is widely used to obtain high quality at a low bit rate. The bit allocation technique that minimizes the distortion due to encoding is allocation in accordance with the signal power of each frequency band, and bit allocation that takes the human sense of hearing into consideration is also done.
On the other hand, there is a technique for improving the quality of a frequency band(s) with a very small number of allocated bits. Patent Literature 1 discloses a technique that makes approximation of a transform coefficient(s) in a frequency band(s) where the number of allocated bits is smaller than a specified threshold to a transform coefficient(s) in another frequency band(s). Patent Literature 2 discloses a technique that generates a pseudo-noise signal and a technique that reproduces a signal with a component that is not quantized to zero in another frequency band(s), for a component that is quantized to zero because of a small power in a frequency band(s).
Further, in consideration of the fact that the power of an audio signal and an acoustic signal is generally higher in a low frequency band(s) than in a high frequency band(s), which has a significant effect on the subjective quality, bandwidth extension that generates a high frequency band(s) of an input signal by using an encoded low frequency band(s) is widely used. Because the bandwidth extension can generate a high frequency band(s) with a small number of bits, it is possible to obtain high quality at a low bit rate. Patent Literature 3 discloses a technique that generates a high frequency band(s) by reproducing the spectrum of a low frequency band(s) in a high frequency band(s) and then adjusting the spectrum shape based on information concerning the characteristics of the high frequency band(s) spectrum transmitted from an encoder.